3 Answers
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Usually, when the phrase small-town charm is used, there is an implication that the charm comes from the relaxed, genuine or unpretentious characteristic of small towns, as well as the fact that small towns can be cute or quirky in a way that big cities can't.

This tourism website encouraging people to visit small towns in Minnesota, and this article describing various charming features of a small town in Ohio, provide illustrative examples of how small-town charm is used.

Due to the characteristics of small towns I listed above, the phrase could also evoke connotations of being uneducated, which the writer in your excerpt is specifically contradicting. You could paraphrase the excerpt something like this:

Despite the light / casual / unpretentious nature of his writing, Tim is undoubtedly an expert.

I just perused several Google hits (in books and on the web), and I can't find where this phrase is used to convey the notion of "uneducated." (Just because people have "small town charm" doesn't mean they are "country bumpkins." The term means "quaint," not "ignorant.") When the O.P. says this was found "in a book," I assume that means in a back-cover review of a technical book. As you said, it means, "This book is an easy read, but don't let that fool you into thinking it won't have any hard value. This guy writes like a friendly neighbor in a small town, but he seriously knows his stuff."
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J.R.Sep 21 '12 at 9:01

This except is truly from a book review. Your comment is very good and Thanks!
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LiuSep 21 '12 at 9:14

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@J.R.: I don't know if you watched the Nixon impeachment hearings (I did), but Senator Sam Ervin (D-NC) was a perfect example of an in-the-wild embodiment of "small-town charm": "Ervin's description of himself as a 'simple country lawyer' has become synonymous with self-deprecation by those who are actually savvy. The Andy Griffith character in Matlock often described himself as a simple lawyer before cleverly solving a case." link
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user21497Sep 21 '12 at 9:28

@BillFranke That's a good example - I always take the phrase to mean that there's a bit of a smokescreen at work i.e. the small town charm is (partially) an act designed to fool people into dropping their guard a little. But maybe I'm being overly cynical...
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tinydSep 21 '12 at 9:38